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哲學功課: Problem of Induction

David Hume has introduced one of the problems in epistemology that baffled many philosophers for ages. In his argument about the problem of induction, he made the claim that inductive inference cannot be rationally justify. Many philosophers had proposed solutions to this problem, such as Hume’s own psychological solution and Kant’s synthetic a priori knowledge solution. However none of them can solve the problem of induction with a satisfaction. This problem is finally resolved by 20th century philosopher Karl Popper by introducing a paradigm shift in the definition of knowledge. This article will first outline Hume’s claim about the problem of induction and the implication of this problem, then it will present Popper’s solution to this problem.

The problem of induction arrived from Hume’s position as an empirical philosopher. Hume believes the source of human knowledge is experience from outside world instead of from reason within our mind. He uses Hume’s fork to divide all human knowledge into two groups, either “relations of ideas” or “matter of facts”. The “relations of ideas” is a prior proposition that can be verified with reason alone. For example, “a circle has no angle” and “pork is pig meat”. The “matter of facts” is a posterior proposition that can only be verified by experience and observations. For example, “the sun always rises from the east” and “it has 400cm of snow in Whistler”. The cause and effect of “matter of facts” are the only concerning matter in human knowledge. Every piece of information in the “matter of facts” has a cause. We know it is going to rain from the dark clouds in the sky. We know that we can take sky train and bus from downtown Vancouver to SFU from the transmit map. Therefore human does not have intelligent without understand the causality between difference pieces of knowledge.

Hume points out the causal connection between two events are discovered by experience but not by reason. Thought induction, we can establish the connection between two events. Induction is generalization work by inferring a claim about an entire population of objects from data about a sample of those objects. We have observed many people die from eating cyanide, so that we can conclude cyanide kills people. However Hume claims that we didn’t actually observe the casual connection between the two events. We only observe a sequence of two events happens one after another all the time and come to the conclusion that there is a causal relationship between them. If we in fact we cannot assert the one event will always leads to another, how can claim we have know anything at all. For example, for many years all birds observed by men have wings. We took it for granted that birds must have wings, until we discover the wingless kiwi bird in New Zealand. Therefore no matter how many winged birds we have observed, we cannot guarantee all birds have wings. The same principle can apply to any other knowledge we have. How can we prove that we will have a full moon next month? We merely expect future events will follow the past experience. We are just assuming the external world is uniform and consistence over time so that all natural phenomena are expected to repeat themselves. This is circular reasoning that we beg the question by assuming the answer we are looking for. Since all our knowledge is based on this assumption that we cannot rationally justify, Hume has shown that we in fact don’t know anything.

Karl Popper is probably the most notable philosopher in the 20th century. His philosophy works changed how the field of epistemology perceives knowledge. For the past two hundred years, the laws of physics discovered by Newton have shaped how philosophers viewed science and knowledge. Science facts are the most reliable form of knowledge in human society. Once a scientific fact is discovered and verified by rigorous experiments, it is thought that the science theory or law will not change. Many great philosophers in the modern age believe that the scope of human knowledge will expand over the time as we discover more scientific facts and add to the existing pool of knowledge. However at the turn of the 20th century, the discovery of a genius scientist Albert Einstein has shaken the foundation of the Newtonian world of physics. His famously relativity theory is compatibility with Newton’s theory. Einstein’s theory is confirmed by the eclipse observations to show that light from distant stars can be bended by the sun’s gravitational field. The consequence of this discovery to philosophy makes us reconsider our understanding of knowledge. We can no longer guarantee the certainty of what we know as any piece of knowledge can call in to question and later shown that it is wrong after all.

Popper’s philosophy shines new lights on the view of science by replacing the traditional static view of science with a more dynamic approach. First he begins with identifying the difference between science and pseudo-science, and defines the properties of a proper scientific theory. Human develop theories to explain the cause and effect between difference events from our experience and attempted to predict the expected result of similar events. Popper thinks that what makes a theory science is not whether the theory is verified to be true nor it is good. A scientific theory has to be falsifiable, namely stated the hypothetical condition that this theory is no longer valid. For example, creationism is not a scientific theory because there is no observation or test could falsify the claim that God created the world in literally 7 days, provided that God can fake any evidences showing the Earth is several million years old . On the other hand, the theory of evolution is science because we can prove it wrong if one day God or some aliens shows up and tell us they had indeed created the Earth. However, Popper didn’t go as extreme as Hume claiming all metaphysics or pseudo science theories meaningless.

Popper has changed the concept of knowledge in philosophy by realizing that there is no certainty in science. Instead he said “science is perhaps the only human activity in which errors are systemically criticized and in time corrected”. Science not only adds new knowledge to the domain, it also replaces existing theories with better ones. It is impossible to prove a scientific theory is ultimately true, we can only claimed that the theory has a tentative status quo, until new there are challenges. When new challenges arise, the old theory either have to be revised to cope with the new findings or limited its scope of assertion to remain useful. The progress of science comes from trial and errors, the continuous regression of hypothesis and observation. Every theory in science not secure, they are open for revision or rejection. All scientific knowledge could probably be false, yet it aspires eventually to the truth. Newton’s theory classical physics is still valid over in the everyday life. Its prediction breaks down when comes to the world of very large scale where Einstein’s relativity theory supplement and correct Newton’s theory. Then along come the quantum theory that is incompatible with both Newton’s and Einstein’s theory on very small scale. The three contradicting theories cannot all be right at the same time. In fact, it is very likely that all of them are wrong. However each theory is served as a stepping stone for a more accurate theory to describe our external world. Although Hume showed that it is not possible to infer a theory from induction of observations, but this does not affect the possibility or falsifying a theory from new observations. Therefore, Popper has solved by problem of induction by changing our concept of knowledge to a more practical and realistic view. Instead of knowing more about knowledge with absolute certainty, we are getting closer to the true nature of knowledge by reducing the degree of uncertainty.

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